On Tuesday, we told you about the 70 activists who poured onto two nuclear sites in Sweden in an effort to show how lax the security is at these plants.

We didn’t tell you that at least six of them hid overnight at two of the plants: four at Ringhals and two at Forsmark.

They evaded security all night, and were only discovered when Greenpeace Sweden phoned the media early this morning to reveal their presence at the plants. This is despite the fact the operator Vattenfall said yesterday that “security had worked exactly as intended”. Oh dear.

One of the overnighters was Greenpeace International energy campaigner Lauri Myllyvirta. Here’s what he wrote about the experience:

“I’ve spent the night on the roof of the Swedish nuclear power plant Ringhals. More than 24 hours – and I’m only out because we chose to reveal our presence here.

That shows me, how alarmingly easy it is to access the vital systems that supply the reactor cooling systems with power.

The recent report on the EU stress tests revealed that in some Swedish reactors, a loss of power to the reactor would lead to a nuclear accident in just one hour.

I took part in this demonstration to draw attention to how little nuclear companies care about the health and safety of people, and how little they do to protect reactors from accidents.

The gaps in safety recently revealed about Swedish nuclear reactors are an absolute disgrace and a cause for alarm. Nuclear operators have not prepared for obstruction of seawater cooling, for snowfall or earthquakes of a magnitude that can occur in Sweden.

The failure of back-up generators and several other safety systems in Forsmark in 2006 were some of the most serious near misses since 2000, before the Fukushima disaster.

The authorities have tried to access the safety violations for years through soft measures: special surveillance, reports, investigations and revised protocols.

Still, blatant security problems continue. The latest evidence was a fire at Ringhals just a few months ago due to a rushed start-up procedure. An intentional cover-up followed.

The only way to end this continuous cutting of corners before a serious accident happens is for violations to have clear consequences: the offending operators need to have their reactors taken offline, at least until the worst issues have been addressed.

The Swedish environmental minister has called the operators in Sweden, E.oN and Vattenfall, to her office today, based on this Greenpeace action, and she has publicly threatened them with revoking the operational license permit if they cannot fix the problems we have highlighted. I hope she sticks to that threat and free Sweden from dangerous, expensive and unnecessary nuclear power."

We wonder what Sweden’s environment minister is thinking today, now that she has learned about Greenpeace activists evading security at two nuclear sites for more than a day.

UPDATE: It turns out there were two more activists than the nuclear facility operator thought! They were never found, but turned themselves in after 38 hours.